Polymerization | Mechanism of polymerization | Degree of polymerization

Polymerization

The process of converting a monomer or a mix of monomers into a polymer is named polymerization. It is a process during which relatively small molecules, called monomers, chemically combine to make a really large chainlike or network molecule called a polymer. Monomer molecules can all be the same or they can represent two, three, or more different compounds. Typically, to make a product you need to combine at least 100 monomer-molecules that have some unique physical properties such as elasticity, high tensile strength, or the ability to form fibers that separate polymers from substances made up of small and simple molecules; Often, several thousand monomer units are integrated into a single molecule of polymer. The formation of stable covalent chemical bonds between monomers set polymerization in addition to other processes like crystallization, where large amounts of molecules combine under the influence of weak intermolecular energy.

The two classes of polymerization are usually distinguished. In condensation polymerization, water is often present with the formation of molecules of several common compounds at each step of the process. In addition polymerization, monomers react to polymer formation without forming by-products. Addition polymerizations are usually conducted in the presence of catalysts, which in some cases control the structural details that have a significant effect on the properties of the polymer.

Mechanism of polymerization

The mechanism of polymerization process utilized in polymer synthesis allows the next classification into two main classes, namely, addition and condensation polymers. The former type is produced by the repetition and sequential addition of monomers without damaging a small molecule during the process. Therefore, no by-products are formed and the repeating units of additional polymers have the same formula that was used to make them, like alkenes or virtually-substituted alkene monomers. These addition reactions follow a step-by-step system involving reactive mediators such as radicals or ions that help to convert monomer pi bonds into polymer sigma bonds. The four different polymerization techniques used in the synthesis of additive polymers are (i) radical polymerization, (ii) cationic polymerization, (iii) anionic polymerization, and (iv) coordination catalytic polymerization.

Mechanism of polymerization


The process of radical polymerization follows three steps, initiation, propagation, and termination. During initiation, a molecule called the radical initiator splits into a thermal or photolytically free radical. A radical alkene then attacks the pi bond in the monomer to form a covalent bond with one of the carbon atoms and turns the other into an active one. The propagation stage then becomes a chain by continuously adding more monomers later on. Termination of chain growth eventually occurs when radical chains combine or participate in the unnecessary reactions involved by pulling hydrogen from another radical chain. Cationic and anionic polymerization follows a similar path overall as their promoters are strong acids and Lewis acids (to convert an alkene into cation), or strong bases, alkaline metals, and organolithium compounds (to convert an alkyne monomer to anion). Catalytic polymerization, known as the Ziegler-Natta polymerization technique, employs catalysts that are complex transition metal-based synthesis complexes derived from transition metal halides and organometallic reagents.

Free radical


The second class of polymers consisting of a large number of highly useful materials is those produced by the polycondensation of monomers combined with a single structure. Condensation polymers are bi-functional in monomers that result in the conversion of functional groups so that each monomer can connect with two others. Polymerization usually leads to the loss of a small molecule that can be in the form of water, gas, or salt. A well-known condensation polymer is Nylon 6,6, a polyamide made from adipic acid (a dicarboxylic acid) and 1,6-hexamethylenediamine, a combination of which destroys water molecules. Nylon 6,6 finds numerous applications in the production of clothing, cooking utensils, carpets, fishing lines, and much more. As polycondensation tends to be slower than polyaddition, the reaction often needs to be heated. A direct consequence of slow polymerization is usually the formation of low molecular-weight polymers. The concentration of high crystals with high tensile strength results from strong interchain interactions when polar functional groups are present along the chain.

Degree of polymerization

The degree of polymerization can be defined as the frequency of the repetition units present in the polymer. For example, if a polymer (p) is composed of 5 numbers of monomers (M), its polymerization degree will be 5.

Further, you'll calculate its degree using the steps mentioned below-

You must first enter the chemical formula of the polymer. For example, consider tetrafluoroethylene [- (CF2-CF2) n -]. The first bound element refers to the monomer unit.

Next, you need to collect the atomic mass of an element that forms a monomer. In this case, carbon and fluorine are involved. From the periodic table, you need to examine the atomic mass of these two elements. The atomic masses of fluorine and carbon are 19 and 12, respectively.

Calculate the molecular weight of the monomer using the following steps.

To calculate the molecular weight, you need to multiply the atomic mass by the number proportional to the quality of the atomic mass (carbon or fluorine atom).

Add both products to get molecular weight. For tetrafluoroethylene, it's (19 x 4) + (12 x 2) = 100.

Finally, you need to divide the molecular mass of the polymer by the calculated molecular weight of the monomer. For example, the polymerization degree of tetrafluoroethylene would be 1200 if its molecular number was 120,000.

Thus, the degree of polymerization formula can be defined as the ratio between the molecular mass of the polymer and the molecular weight of the monomer.

Some examples of a polymerization

N-Vinylpyrrolidone (NVP)

An organic compound N-vinyl pyrrolidone (NVP) containing a 5-member lactam linked with a vinyl group. It is a colorless liquid although commercial samples may be yellow in color. It is produced by industrial-based 2-pyrrolidone vinyl, i.e. a base-catalytic reaction with acetylene. It is a precursor to an important synthetic material polyvinylpyrrolidone (PVP). NVP monomers are commonly used as a reactive dilute between ultraviolet and electron-beam curing polymers applied as ink, coating, and adhesive.

p-methyl styrene polymerization

The synthesis of stereoregular styrenic polymers in single-site catalysts has received considerable attention in recent decades; However, fully isotactic poly (P-methyl styrene) is rarely known for tailoring. It has been shown here that the isospecific coordination polymerization of p-methyl styrene can be achieved in the presence of the catalyst dichloro[1,4-dithiabutanediyl-2,20-bis(4,6-di-tert-butyl-phenoxy)] titanium activated by methyl aluminoxane. Furthermore, p-methyl styrene has been assumed to be non-crystallized for decades, as X-ray diffraction and differential scanning calorimeter measurements of the powder sample failed to reveal a well-defined crystal structure. However, dendritic crystals of P-methyl styrene were successfully made from dilute solutions by carefully controlling the evaporation rate of the solvent. Crystal morphology was studied by optical microscopy and atomic force microscopy. In situ heating tests of dendritic crystals allow us to measure the melting temperature of p-methyl styrene crystals. In addition, a vapor annealing process is performed to prepare multilamellar crystals for X-ray scattering measurements at small angles to mark the spacing and orientation of the formed crystalline lamellae.

N-carboxy anhydride polymerization

N-carboxy anhydride (NCA) polymerization is the most widely used polymerization technique to make synthetic polypeptides and polypeptide-based block copolymers on the multigram scale. NCA polymerizations were initiated using a variety of different nucleophiles and bases, the most common being primary amines and alkoxide anions. The primary amines, being more nucleophilic than the basics, are good general initiators for the polymerization of NCA monomers. Compared to nucleophilic, tertiary amines, alkoxides, and other precursors have found use because they are able to prepare very high-molecular-weight polymers in some cases where the primary amine cannot initialize. The optimal polymerization conditions are often determined faithfully for each NCA and so there is no universal indicator or condition that can produce a high polymer from any monomer. This is partly due to the different properties of the individual NCAs and their polymers (e.g., solubility) but is strongly related to the side effects that occur during polymerization.

Xylan polymerization

Xylan is a type of hemicellulose that represents the third most abundant biopolymer in the world. It is found in plants, dicot second cell walls, and all cell walls of grass. Xylans are polysaccharides produced from β-1,4-linked xylose residues with side branches of α-arabinofuranose or α-glucuronic acids that in some cases contribute to the crucible connection with cellulose microfibrils and lignin through ferulic acid residues. Based on the substituted group, Xylan can be classified into three classes i) Glucuronoxylan (GX) ii) Neutral Arabinoxylan (AX), and iii) Glucuronoarabinoxylan (GAX).

Xylan plays an important role in plant cell wall integrity and enhances cell wall recovery in enzymatic digestion; Thus, they help to protect plants from herbs and germs. Xylan also plays an important role in the growth and development of trees. The quality of cereal flour and the hardness of the dough are largely influenced by the amount of xylan, which plays an important role in the bread industry. The main ingredient in xylan can be converted to xylitol used as a natural food sweetener, which helps reduce tooth decay and acts as a sugar substitute for diabetics.

N-vinylcarbazole polymerization

N-vinylcarbazole is an organic compound used as a monomer in the manufacture of polyvinyl carbazole, a conductive polymer, where the conductivity is photon-dependent. The compound is used in the photoreceptors of photocopiers. Upon contact with γ-irradiation, N-vinyl carbazole undergoes solid-state polymerization. It is produced by the vinylation of carbazole with acetylene in the presence of the base.

A crystal of N-vinyl carbazole was polymerized in water suspended by a redox catalyst and polyvinyl carbazole was found. Polymerization progresses rapidly above 40°C without solid-state bringing time. The molecular weight of the polymer increased with a decrease in catalytic density and an increase in temperature. Observations of partially polymerized crystals through a polarizing microscope showed that the polymerization progressed beyond the surface of the monomer crystal and that birefringence was observed in the polymer layer. X-ray compression studies showed that the polymer was crystalline.

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